A redox-flow type battery is known as that for coping with the change of demand of electric power by storing excess electric power in the nighttime and supplying it according to increase in demand in the daytime. In this redox-flow type battery, electrochemical energy conversion is carried out in the cells of the battery system supplying active materials from the outside. The electrochemical reaction at this time is ordinarily a heterogenous reaction occurring on the surface of an electrode, and the electrochemical reaction field is generally two-dimensional. Therefore, the old redox-flow type battery had such a defect that the reaction density per unit volume of an electrolytic cell was small.
Then, in order to increase the reaction density per unit volume, that is, the current density, three-dimensionalization of the electrochemical reaction field has been practiced and textile fabrics such as nonwoven fabrics or woven fabrics of carbon fibers are used as an electrode thereof.
Typical examples of flow-through type electrolytic cell having a three-dimensional electrode are schematically shown in the accompanying FIGS. 5 (a), (b) and (c). The cell comprises a membrane 1, flow paths of an solution of an active material 2, collectors 3 and a three-dimensional electrode 4 of nonwoven cloths, woven cloths or the like made of carbon fibers. For example, tanks of the solution of the active material are disposed upper and lower side and right and left side of the cell shown in these drawings, respectively. In the nighttime, the solution of the active material flows from the lower tank to the upper tank as shown by the arrows and the solution passes through the texture of the three dimensional electrode 4 to effect the electrochemical reaction to charge with electricity. In the daytime, the solution flows in the reverse direction to supply electricity.
In Japanese Patent Kokai No. 59-119680, there is disclosed a flow-through type electrolytic cell having a three-dimensional electrode of knitted fabrics of carbon fibers as shown in the accompanying FIG. 4. The cell comprises collectors 5, frame type spacers 6 formed by thin insulating plates and a membrane 7 such as an ion exchange membrane, a porous membrane or the like. In the inner space of the spacer 6, there is provided a three-dimensional electrode 8 comprising warp knitted fabric of carbon fibers in rib stitch, pearl stitch, tuck stitch, float stitch, interlock stitch or the like and weft knitted fabric of carbon fibers in two needle stitch, perlin stitch, double denbigh stitch, double half stitch, back half stitch and the like so that gaps 6a and 6b are formed at both upper and lower parts of the electrode. The spacers 6 and the collectors 5 are overlaid on both sides of the membrane 7 and flow paths 9 for an solution of an active material are fixed on both upper and lower parts of the collectors 5 so that the openings thereof communicates to the gaps 6a and 6b, respectively. The solution of the active material flows in the upper or lower direction through the texture of the three-dimensional electrode 8.
The warp knitted fabric and weft knitted used in the above three-dimensional electrode 8 are obtained by interweaving knitted cloths using yarns of man-made or synthetic fibers such as regenerated cellulose fibers and the like and then carbonizing them. However, since the above knitted cloths are interwoven with one kind of yarns, their textures are relatively fine such as rib stitch, double denbigh stitch and the like as described above. Thus, all yarns bend in the same way to form loops, go back and forth in the thickness direction, and tangles with each other, which forms small concavo-convex parts all over the surface almost uniformy and complicatedly bending voids in the texture. For these reasons, when the solution of the active material flows, pressure drop becomes great. Therefore, a large amount of energy is required for pumping the solution, which results in decrease in total energy efficiency of a battery. By the way, use of unwoven cloth for the three-dimensional electrode 8 is also known. However, in this case, since fibers cross with each other at points and contact area between fibers is small, there is a problem that electric resistance as an aggregate becomes larger and also cycle change of resistance with time is liable to become larger. On the other hand, when woven cloth of the above yarns is used, there is a problem that crossing and contacting area of yarns become too large and a larger reaction field is not expected in spite of requiring much yarns. Further, since an aqueous solution of an active material can not pass through the inside the texture but pass over the surface and the back surface of the woven cloth, the inside of the texture can not be worked as the reaction field.